The present invention relates to passive loop-through of signals in signal processing equipment, and more particularly to an improved power-off, loop-through return-loss with negative capacitance for a video monitoring instrument.
Current analog and some digital video inputs of waveform monitors and other video processing equipment are required to have a passive loop-through characteristic to allow cascading of equipment on a single video cable run, This reduces the need for analog and digital distribution amplifiers (DAs) and allows a waveform monitor (WFM) access to the physical signalxe2x80x94eye-pattern, jitter, etc.xe2x80x94passed down-stream to subsequent video equipment inputs. It is important that the WFM and other video devices be non-invasive to the video signal even when the WFM is powered off. This requires a high, large-signal return-loss, typically greater than 40 dB, in both the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d power condition. A robust method is to isolate a high impedance input amplifier with a resistor to provide good return-loss and electro-static discharge (ESD) protection. Typically the resistor must be greater than or equal to 7500 ohms to attain the desired return-loss and ESD protection. However this creates a dominant pole at 2-5 MHz with the electronic circuit board (ECB) run capacitance and device input capacitance. The bandwidth is too low.
What is desired is a way to allow the resistor to be greater than 7500 ohms needed to meet the 40 dB return-loss specification and not reduce the bandwidth due to the stray input capacitance below a desired 100 MHz or higher.
Accordingly the present invention provides an improved power-off, loop-through return-loss for an instrument by adding a positive feedback capacitor, corresponding to negative input capacitance, to cancel stray capacitance and extend a dominant input pole to 100 MHz or higher.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.